1. Field of the Invention
The present invention relates, in general, to a spacer grid used for placing and supporting a plurality of nuclear fuel rods within a nuclear fuel assembly and, more particularly, to a duct-type spacer grid consisting of a plurality of duct-shaped grid elements individually having an octagonal cross-section. The grid is also designed to have a plurality of swirl flow vanes at the top of each grid element.
2. Description of the Prior Art
As shown in FIG. 1, a conventional nuclear fuel assembly 100 typically comprises a plurality of spacer grids 110, a bottom nozzle 101, a top nozzle 102, a plurality of guide tubes 103, and a plurality of elongated fuel rods 106.
In the above fuel assembly 100, the elongated fuel rods 106 are regularly and arranged in parallel to form a structure having a square cross-section while being placed and supported by the spacer grids 110. Each of the grids 110 is fabricated by assembling a plurality of intersecting inner strips into an egg-crate pattern. The intersecting inner strips are also welded together at their intersections.
As best seen in FIGS. 2 and 3, a plurality of inner strips 113 intersect each other to form a plurality of four-walled cells 108 having a square cross-section. In each of the four-walled cells 108, two springs 114 are provided on the interior of two neighboring walls, while two dimples 115 are provided on the interior of the opposite two walls. The strength of the two dimples 115 is higher than the two springs 114. Both the two springs 114 and the two dimples 115 are used for supporting an elongated fuel rod 106 within each cell 108.
In the above fuel assembly 100, it is necessary for the springs 114 and the dimples 115 to effectively support the fuel rods 106 while restricting undesirable movement of the rods 106 even when the assembly 100 is impacted by any external force applied in an axial direction A, a radial direction B and/or a rotating direction C. Such an external force, applied to the assembly 100, may be caused by the coolant flow, an earthquake or any unexpected external impact. In addition, the grid structure 111, consisting of the intersecting inner strips 113, has to maintain the originally designed configuration of the cells 108 even when a lateral impact is applied to a sidewall of the grid 110. In the fuel assembly 100, the spring force of the grid 110 may be gradually reduced due to neutron irradiation of the assembly 110. The spacer grid 110 has to be designed to maintain an effective spring force capable of continuing elastic contact of the springs 114 with a fuel rod 106 until the existing rod 106 is changed with a new one.
In the above fuel assembly 100, the fuel rods 106 may be grown in an axial direction A due to the neutron irradiation, and so the grids 110 have to be designed to appropriately support the rods 106 while allowing such an axial growth of the rods 106. However, when the spring force of the grids 110 undesirably exceeds a reference level, the fuel rods 106 may be prevented from being grown in the axial direction A. This sometimes results in a bending of the fuel rods 106. When the fuel rods 106 are undesirably bent as described above, it is difficult to secure a subchannel 107 within the fuel assembly 100. This deteriorates the cooling performance of the assembly 100. FIG. 4 shows a subchannel 107, formed by four fuel rods 106. On the other hand, when the spring force of the grid 110 is less than the reference level, the grids 110 may fail to effectively place or support the fuel rods 106 within the assembly 100. This finally results in vibration or fretting wear of the fuel rods 106, thus severely damaging the rods 106.
As well known to those skilled in the art, the power output from a nuclear reactor is partially used as an energy source for causing the coolant to effectively flow within the reactor core. The amount of power, required to cause the coolant flow within the core, is determined by a hydraulic resistance in the flow paths. In a conventional nuclear fuel assembly 100, the flow paths comprise a main flow path and a sub-flow path. When the flow paths are designed having a shape which disturbs the coolant flow, a large amount of power has to be consumed to cause the coolant flow. On the other hand, when the flow paths are designed having a streamline shape, a small amount of power is needed to cause the coolant flow. It is necessary to make the passages effectively cause the coolant flow using a small amount of power by reducing the hydraulic resistance.
A typical spacer grid for nuclear fuel assemblies, used in light water reactors, may be referred to U.S. Pat. No. 3,395,077. Another conventional spacer grid, having a specifically designed inner strip and a fuel rod support spring, may be referred to U.S. Pat. Nos. 4,426,355, 4,726,926, 4,803,043 or 4,888,152.
In the spacer grid of U.S. Pat. No. 4,426,355, the inner strips are corrugated to form a plurality of wavy dimples at regularly spaced positions. In the spacer grid of U.S. Pat. No. 4,726,926, a plurality of thin and narrow inner strips intersect each other prior to being welded together at their intersections, thus forming a grid structure. After the grid structure is formed by the intersecting inner strips, the strips are appropriately deformed to form a plurality of flow paths, springs and dimples. In the spacer grid of U.S. Pat. No. 4,803,043, the springs of the inner strips are positioned to be diagonally opposite to each other, thus having an increased effective spring length. In the above-mentioned spacer grids, each grid consists of a plurality of intersecting inner strips. In such a spacer grid having the intersecting inner strips, the inner strips pass across the subchannel having a high flow rate. This type of spacer grid is thus problematic in that it undesirably results in an increase in pressure loss.
On the other hand, U.S. Pat. No. 4,888,152 discloses a ring-type spacer grid that comprises a plurality of ducts shaped grid elements individually having a square cross-section. In order to form a spacer grid, the grid elements are slitted at appropriate portions and are intersected to each other in a way such that the grid elements form a grid structure arranged in pararell. Such a ring-type spacer grid does not pass across the subchannel different from the grids having the inner strips. However, this ring-type grid is problematic in that the fuel rods are placed and supported by rigid corners of the grid elements, thus being apt to be severely damaged when the fuel rods have vibrated.
As well known to those skilled in the art, there is a difference between the output powers of the fuel rods within a reactor core due to a nonuniform distribution of neutron flux. Therefore, a subchannel, adjacent to a fuel rod having a high thermal power output, may be highly increased in enthalpy comparing with the other neighboring subchannels. In accordance with an increase in the power output of the fuel rods, coolant in the subchannel having the high enthalpy rise, may be boiled prior to cooling within the other subchannels. There primarily occurs a nucleate boiling and secondarily a film boiling of water within the subchannel having the high enthalpy rise. When a film boiling occurs, a bubble film is formed on a fuel rod surface. Such a bubble film decreases heat transfer from the fuel rod surface to the coolant, thus increasing the temperature of the cladding surface of the fuel rod. Such an increased temperature of the cladding surface results in a partial thermal stress on the cladding. When the temperature of the cladding is further increased, both the cladding may be melted. It is thus necessary to limitedly operate the reactor core in a way such that any film boiling does not occur in the subchannels. Such an undesirable phenomenon, caused by film boiling in the subchannel, is a so-called xe2x80x9cDeparture from Nucleate Boiling(DNB)xe2x80x9d in the field. The DNB is affected by the intervals between fuel rods, system pressure, thermal power output, enthalpy rise and core inlet coolant temperature. In order to allow a nuclear fuel assembly to output a high power while being free from such DNB, it is necessary to make a uniform temperature distribution of coolant within a nuclear reactor. When such a desired uniform temperature distribution of coolant is accomplished, the coolant is prevented from being partially overheated while maximizing the thermal output. Such a uniform temperature distribution of coolant may be accomplished by effectively mixing the coolant within each subchannel or between a plurality of subchannels of a fuel assembly. In order to mix the coolant within a fuel assembly as described above, a mixing vane, or an integrated mixing device, may be provided on the top of the strips of the spacer grid. The above mixing vane structure may be designed to cause a cross flow of coolant from a subchannel to a neighboring subchannel. Alternatively, the mixing vane structure may be designed to cause a swirling flow of coolant within a subchannel or around a fuel rod.
U.S. Pat. No. 4,879,090 discloses a typical vane structure for mixing the coolant within a nuclear fuel assembly. Another type of mixing vane structure for nuclear fuel assemblies may be referred to U.S. Pat. Nos. 5,299,245, 5,110,539 or 5,440,599. On the other hand, U.S. Pat. No. 4,726,926 discloses a specifically designed mixing device of the flow deflector type.
The mixing vane structure of U.S. Pat. No. 5,299,245 comprises four swirl flow vanes, which have a blade shape with a slitted end and are arranged within each subchannel. The four vanes of such a blade type are designed to swirl the coolant within a subchannel. However, this vane structure is problematic in that since the vane, arranged in the center of the subchannel, disturbs the smooth flow of coolant. This increases pressure loss in the fuel assembly. On the other hand, the mixing vane structure of U.S. Pat. No. 5,110,539 comprises two swirl flow vanes, which have a blade shape with a slitted end and are arranged in the center of each subchannel. The two vanes of such a blade type are designed to swirl the coolant within the subchannel. However, this vane structure is problematic in that the vanes may be easily damaged by fuel rod insertion. The mixing vane structure of U.S. Pat. No. 5,440,599 comprises two swirl flow vanes, which are laterally supported and are arranged within each subchannel. The two laterally supported vanes are designed to move coolant from a subchannel to a neighboring subchannel. However, this vane structure is problematic in that the lateral flow of coolant comes into collision with the main flow in subchannel, thus being disturbed by the main flow of coolant. This finally deteriorates the coolant mixing effect of the grid. The vane structure, disclosed in U.S. Pat. No. 4,726,926, comprises a plurality of flow deflector with a bent end. The flow deflectors are designed to move the coolant from gap channels, defined by the flow path between fuel rods, to the center of the subchannel. However, this flow deflector is problematic in that the deflected coolant comes into collision with coolant from an opposite deflector, thus reducing the coolant mixing effect caused by the cross flow.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a duct-type spacer grid for nuclear fuel assemblies, which consists of a plurality of duct-shaped grid elements individually having an octagonal cross-section capable of effectively resisting against a lateral impact, and which does not pass across a subchannel, thus reducing pressure loss.
Another object of the present invention is to provide a duct-type spacer grid for nuclear fuel assemblies, which effectively generates a swirl flow of water within subchannels, thus improving the thermal mixing performance of the fuel assembly.
A further object of the present invention is to provide a duct-type spacer grid for nuclear fuel assemblies, which supports each elongated fuel rod using line contact springs without using any dimple, thus uniformly distributing the spring force on the spring contact area of the fuel rod, thus almost completely preventing damage of the fuel rod due to a fretting wear.
In order to accomplish the above object, the present invention provides a duct-type spacer grid for placing and supporting a plurality of elongated fuel rods within a nuclear fuel assembly, comprising: a plurality of duct-shaped grid having an individual regular polygonal cross-section, the grid elements being closely arranged in parallel and assembled together to form a plurality of main flow paths between them, the main flow paths being used for allowing fuel rod coolant to pass through and having an individual polygonal cross-section, each of the grid elements including: a plurality of spring windows formed on a plurality of sidewalls of each polygonal grid element; a surface line spring provided within each of the spring windows while being bent toward the center of each grid element at a central portion thereof, thus elastically supporting an external surface of a fuel rod inserted into each grid element; and a plurality of swirl flow vanes axially extending from a top of each grid element and having different heights or the same height, each of the vanes being bent twice outwardly from each grid element toward the center of an associated subchannel and generating a swirl flow of coolant.
Each of the duct-shaped grid elements forms a main flow path thereby allowing coolant to pass through the main flow paths. The duct-shaped grid elements are arranged in parallel while forming a regular angle between them. The grid elements are, thereafter, welded together at their upper and lower area of wall and at one or more points at each of the upper and lower areas of the wall.
The line contact spring is bent thoroughly from the sidewall of each grid element toward the center of the grid element. The spring also forms a contact surface when it is brought into line contact with the external surface of the fuel rod. Each of the spring windows forms a passage used for allowing coolant to pass through, and is axially formed on an associated sidewall of each grid element while being parallel to the axis of the grid element, thus having a longitudinal shape.
Each of the swirl flow vanes is primarily bent outwardly to form a sub-blade and is secondarily bent outwardly to form a main-blade. The two blades are used for generating a swirl flow of coolant. The sub-blade is outwardly bent at an acute angle relative to the grid element, with the main blade being outwardly bent from the inclined portion of the sub-blade toward the center of an associated main flow path.